>On Wed, Oct 30, 2002 at 06:40:06PM +0100, Martin Regner wrote:
>> The other problem I had was related to that most outgoing packets over a
>> certain size had incorrect checksum that made Ethreal not considering
>> the packets with incorrect checksum when reassembling the tcp data.
>>
>> http://www.ethereal.com/lists/ethereal-users/200210/msg00059.html
>
Guy Harris wrote:
>In that message, you didn't mention that the problem was, as you appear
>to be saying, only a problem with *outgoing* packets.
>
>Given that, I suspect that the interface on which those packets are
>being transmitted does TCP checksum offloading. If you are capturing
>traffic on such an interface, the TCP checksums on outgoing packets will
>probably show up as incorrect in the capture, as the outgoing packets
>are *not* seen by the network interface on which you're capturing;
>instead, the driver internally hands the packet to the capture mechanism
>- but the TCP code won't have set the checksum field on the packet, as
>it'll be relying on the network interface to do so, so the packet
>handed to the capture mechanism, and thus to tcpdump or Ethereal or ...
>will have a bad TCP checksum.
Yes, in my case the problem was due to "checksum offloading" as you write above.
I guess that Mads Nielsen that wrote the message I referred to also had incorrect checksums
due to "offloading".
It was Mads message that made me notice that I needed to turn off the "Check the validity of the TCP checksum" option.
There was some discussion regarding incorrect checksum (e.g. due to "offloading") some days
after Mads message, but it seems that that discussion was in another thread - so I should have
referred to at least one of the messages in that thread also:
http://www.ethereal.com/lists/ethereal-users/200210/msg00070.html
http://www.ethereal.com/lists/ethereal-users/200210/msg00077.html
http://www.ethereal.com/lists/ethereal-users/200210/msg00080.html
http://www.ethereal.com/lists/ethereal-users/200210/threads.html#00072
